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Not Sure what is wrong

When I turn on the main water valve to my irrigation system, zone 1 (valve 1) activates. This happens with my timing system on or off. I cleaned and replaced the valve top and diaphragm with no change. I have unwired and re-wired the system with no change. I have a 4 zone system with a Rainbird SST-600i

In trying to resolve this issue I turned on the water and left it running then turned the controller to zone 1 and pushed the manual "water now" button. I can hear the system click on and when it does the water pressure of zone 1 decreases.

If you've eliminated the electrical, and it sounds like you have, all that is left is the mechanical or pressure. I don't see how pressure could come into this. I'd rebuild the valve. If you are really energetic and have another zone with the same type valve you can swap internals and solenoid and see if the problem follows the parts.

Maybe you have your valves mixed up. Maybe the valve that's stuck on isn't valve number 1. That would explain why the water pressure decreases when you turn on number 1 at the controller. Maybe there's two valves running at once at that point. You possibly looked at the wrong valve.
It sounds like a bad diaphragm or a stuck plunger in the solenoid to me. Possibly debris in the valve keeping the diaphragm from closing.

Took me a day to think of it. You would have thought of it sooner or later. It's hard to tell online sometimes what's what. Maybe he'll come back with the answer for us. I wish more people would do that. A lot of people dont.

Yup. One of the frustrating things here is coming across something really interesting or challenging and not being able to see it. I'd love to be able to say "Heading for the truck. Will be there shortly."